The USD 2.2bn Linthal 2015 project includes steep TBM drives, drilland- blast caverns and access drives within a mountain, cablecar access for major supplies, and even the frequent use of helicopters, all within a picturesque area, of course covered in snow in winter, but also with daily falls of up to 200mm in summer. Such conditions present major challenges for design engineers and contractors.
The purpose of Linthal 2015 is to increase the pumped storage capacity of the 450MW Linth-Limmern hydropower complex by 1000MW. Originally constructed between 1957 and 1968 in the Switzerland Canton of Glarus, Linth-Limmern is operated by Krafttwerke Linth-Limmern (KLL), a partnership of Swiss utility group Axpo (85 per cent) and the canton of Glarus (15 per cent). The new plant will be a major facility to increase out KLL’s peak-load capacity, at the same time generating more income as the pumping is carried out at lower cost periods. This will supplement Switzerland’s usual baseload generating facilities from nuclear and run-of-river hydro plants.
The existing plant comprises four power plant stages between Muttsee and Linthal itself – a difference in elevation of 1800m. The main reservoir capacity is Limmersee, a dammed reservoir that will also form the low point of the new facility. This holds up to 92M m3 of water.
The new development necessitates the construction of an underground powerhouse, transformer cavern and a balancing chamber, all around 600m inside the mountain, as well as associated access tunnels, two TBM-driven headrace tunnels and two tailrace tunnels. These are situated mainly between two mountain lakes used as reservoirs. The lower Limmernsee is at an elevation of 1700m, from which water is pumped to the Mittsee, about 630m higher. The return of the water under gravity head of 560-709m, through two headrace tunnels will be used to drive the turbines in the new power chamber.
In addition, in order to increase the capacity of Muttsee from 9Mm3 to 25 Mm3, raising the potential head by 28m a new 1000m-long, 35m-high dam is being constructed on the edge of the corrie, whilst there will also be other modifications to both intake and outfall structures at both lakes.
Axpo, which serves around three million of Switzerland’s population, has let the work contracts under open tender assessed on bid price and technical capabilities to a consortium of Marti companies, called Arge KW Limmern, for the main power station works. The value of Marti’s contract (Lot A) is USD 727M. Arge Zugangstollen (Limmern-AZL) consortium is driving a steep access tunnel to house a funicular railway for permanent access to the new plant. The participants are Rothpletz, Lienhard & Cie, Wayss & Freytag Ingenieurbau, Barsel, G Lazzarini, Andrea Pitsch and Ragotti+Weber Bau.
Geology
Throughout the extent of the new underground workings the ground is expected to consist of medium-to-hard limestone that is stable and dry. However, as in the rest of Switzerland, the limestone adopt a karstic structure with caverns and passageways that can allow large amounts of water to flow through the structure.
Access and supply
Not only does the topography mean access to the 20-odd worksites at elevations up to 2500m above sea level is difficult, but the Linthal area itself is also remote, at least in European terms. Thus the transport of heavy tunnelling equipment, such as 36-t drill rigs and TBM components, and a concrete batching plant, has been difficult.
From the base station, every one of the heavy items of tunnelling and construction equipment has to be carried to its required location by two cableways of up to 40-t capacity using special suspension arrangements or standard containers hanging from the Doppelmayr Garaventa cableways, as well as a passenger cabin on each. These were installed at an early stage in the development of the Linthal 2015 project, from 2007, and, compared to road transport in very difficult conditions, are essential and remarkable in that the 40-t capacity is a world record. The lower one, from Tierfehd to Chalchtrittli, will be removed once the project is complete. This rises the loads through 1051m over an inclined length of 1922m. The upper cableway runs from Ochsenstafeli to the upper reservoir and dam site at Muttsee. In this case the vertical lift is 596m over an inclined length of 1771m.
The Marti construction consortium also operates the logistics chain, which includes a number of specialised Terberg CCRT222, 4-wheel-drive Ro-Ro container carriers, and RT (port tractors). Like other equipment they arrived at the Tierfehd base station for the project. Most, particularly smaller equipment arrives by rail in standard 20-FT (6.1m) containers, so the first duty for the container carriers is to transport the containers from the railhead to the base of the first cableway.
Further up inside and around the mountain the Terberg trucks carry out a number of duties between the cableways and ten different worksites, including driving through access tunnels. Thus they take the supply containers to where required, open-pan carrying of aggregates from excavation for concrete production, and even containerised Schwing-Stetter concrete mixers that are used to supply sprayed concrete units. The cableway and Terberg containers transport will have to carry an estimated 100,000t of cement and 40,000t of construction steel to where they are required.
Most of the access tunnels, as well as the caverns, are being driven by drill-andblast using five Atlas Copco drill-rigs. Three of the most recent Boomer E2C design (two drill booms plus basket) using 2238 rock drills, and Marti also has a two boom 281-1B rig. The fifth rig, an L2C, is rented from Atlas Copco. Within Marti’s contract the drill-and-blast tunnels total about 5km in length. Together with the caverns the expected excavation rate is 500-700m3 daily. Sections range from only 5 to 1000m2 for the main power station cavern.
Under a separate contract to the Arge Zugangstollen (AZS-Limmern) consortium an 8.03m-diameter Aker Wirth hard-rock TBM is being used to drive a 4km-long access drive inclined upwards at an incline of 24 per cent to rise from 800m ASL at Tierfehd up to 1800 ASL. This will be used to house a funicular railway for permanent transport to the new installation for maintenance and general operational access, such as to the control centre. The tunnel is being driven up to the main power chamber from, effectively, the ‘base station’ at Tierfehd, which is also the location of another KLL hydropower plant. The open double-gripper TBM with double bracing features a 160m-long back-up system, with anti-slip device, manufactured by Rowa Tunnelling Logistics and weighing 1500t in total. Aker Wirth and Rowa entered into partnership to supply the required equipment assembly.
Aker Wirth says that tunnelling the Limmern will be the largest ever shaft to be completed by a TBM, the previous largest being of 6m diameter. “Aker Wirth’s TBMs have been successfully used in inclined shaft projects since the mid-1960s,” says CEO Christoph Kleuters.
Arge AZS-Linthal construction manager Christian Ris adds, “It has never been possible before to use such a big TBM to excavate such a steep ascending tunnel.”
The Aker Wirth TBM and Rowa backup is required to negotiate a vertical curve of 300m radius to align it with the planned 24 per cent incline. This is aided by the installation of a PPS (Poltinger Precision Systems) TBM guidance system that was fitted and calibrated by PPS on site last September.
Rowa also designed and manufactured a hoist car used to carry supplies to the TBM, and a belt conveyor to remove spoil to the portal. As the TBM progresses, side niches for refuges are excavated by conventional means, as will be a lay-by for the future railway operation half-way along the drive. In a third phase the bore will be lined with cast in situ concrete using the hoist car to supply the concrete mix.
Caverns
The new power cavern is to be 150m long, 57m high and 30m wide to house four turbine generator/pump sets of 250-MW capacity each. A second cavern, measuring 46m high and 22m wide, in parallel and separated from the main cavern by a 59mwide rock pillar, will house transformers and switchgear. The main cavern will also house the main control centre for the new plant.
At a lower level, however, the pillar is penetrated by the tailrace tunnels, and others for cables and access. The Alpenstrom engineering practice of Zurich designed the structure as the best compromise between increased stability and operational efficiency. The results of calculations performed using Examine 2D software were increased to include an additional safety factor to ensure that the new excavations would not displace the walls of the original power cavern unacceptably. As an addedmeasure to improve stress distribution, the walls of the main cavern Slope inwards rather than excavating vertically.
Headraces
There are two headraces to be constructed, particularly in a common large-diameter tunnel about 800m long below the partially drained Muttsee, and then in two separate tunnels, each a kilometre long, to be lined with solid steel to handle the water pressure and ensure water-tightness. A Herrenknecht gripper TBM equipped with VMT laser guidance will excavate these drives. Each is a 5.3m in diameter, and inclined at 40 degrees (87 per cent), thus presenting a major challenge in itself. Although steeply inclined TBM drives have been tackled before, this is the first time that larger-diameter Herrenknecht TBMs have been involved. On such a steep incline there is no room for error, thus placing extra demands for attention on the personnel operating it, and requiring some design adaptations on the TBM itself.
Negotiation of the start of the drives is particularly tricky in that the alignment includes a vertical curve of only 150m radius right at the start of the drive to follow a launch incline of 25 degrees to advance to the main drive’s 40-degree incline in the length of only 50m. The length of the TBM and back-up system is 150m.
VMT’s role in these drives includes the supply of a special SLS-HR (hard-rock) navigation system. This has not been used on an incline as steep as 40 degrees before. Reference surveying practices have to be adapted to the steep working angles, with the survey team carrying out control measurements that can be referenced by the laser guidance system including coordinates in the incline.
The first headrace drive is making good progress it is reported, having advanced 200m since launch in mid-December last year. Completion of the bore is expected around this Easter at a scheduled 22m advance per day, after which the TBM will be dismantled a transferred to the second drive, allowing the first headrace to be lined with steel.
DSD Noell of Germany has won a contract to design, manufacture and install the two penstocks to be installed on the headraces.
Tailraces
Like the access tunnels at power station level, the tailrace drives are being excavated by drill-and-blast. There are two, each 500m long, running from the turbine discharge manifold to Limmernsee, acting as a 2-way water passage for pumping and gravity discharge. As the design pressure is low compared to the headraces, now special steel lining is required.
Support and concrete
Much of the concrete required in the project is produced using aggregates sourced from the excavations, and an underground batching plant.
Initial support is generally by sprayed concrete using Meyco BASF equipment and concrete mixes featuring BASF Construction Chemicals additives. Two Meyco Potenza mobile spraying units have been used in the main cavern for securing initial support, allowing subsequent decisions on the extent of secondary/permanent support required, if any.
In the headrace drives, the TBM leaves exposed rock that receives immediate sprayed concrete support using a Meyco DM dry-mix spraying machine, and a Meyco Rama 6 spraying arm mounted directly on the Herrenknecht TBM.
In one of the access drives, excavated by drill-and-blast, using another Meyco Potenza unit is being used for primary support. This machine will apply about 60 000m3 of sprayed concrete including BASF Meyco SA 166 accelerator and also Rheobuild T3 plasticiser to improve handing of the mix.
The underground batching plant also includes a casting facility for production of segments to be used for lining the inclined drive being driven under separate contract by an Aker Wirth TBM for the permanent funicular railway for power station access. Meyco’s Glenium ACE 30 hyperplasticiser and MicroAir 302 airentraining agent are both used in mix preparation in the batching plant for the casting concrete.
Concrete for spraying is carried to the spraying equipment using the Terberg vehicles mentioned previously carrying containerised Schwing-Stetter concrete mixers to keep the mix agitated.
The fourth Potenza Robojet is being used by Marti around Mast 3 adjacent to the Muttsee for slope stabilisation etc in connection with increasing the capacity of the upper reservoir.
At the lower end of the project Rothpletz Leinhard, part of the Arge ZSL consortium, is using a Meyco Spraymobile together with a Meyco Suprema concrete spraying unit for work around Tierfehd in the valley floor. This is the lower access point for the project and the start-point for the funicular railway drive. Additional wetprocess concrete spraying equipment from Meyco is being used on the Aker- Wirth TBM excavating this drive, again for immediate support of the rock exposed by the TBM as with the Herrenknecht headrace bores.
Schedule
As it’s name suggest, project Linthal 2015 is due for completion in 2015-2016 in two phases. Marti’s work began in October 2009. At the end of last year over 1.5km of drill-and-blast tunnels had been completed and both TBMs were in action. The Herrenknecht headrace drives are scheduled for breakthrough by the end of this year, after which lining and support will be completed and penstocks installed for commissioning in 2015 and 2016.
Diagrammatic plan of the relationships between the upper Muttsee, lower Limmernsee reservoir and access tunnel under separate contract. (The excavations for the existing power plants are omitted for clarity) Construction site at Muttsee for the new dam and water intake Section along to new excavations of Linthal 2015 from Muttsee down to the new power station and water transfer to Limmernsee One of the many rock faces being wired up for blasting seen from an Atlas Copco E2C drill-rig View of the Thierfehd base supply station from up the Garavanta cableway Arge ZSL’s 8.03-m diameter Aker Wirth hard-rock TBM at the village of Thierfehd in the Linth valley with total station used to sent up the PPS TBM guidance system Motorised theodolite prisms mounted on the Aker Wirth TBM as part of the PPS guidance system for inclined drive One of many wheeled loaders used by Marti on the project, a Sandvik Toro mining-type load-haul-dump vehicle in a cavern excavation One Marti’s four Meyco Potenza Robojet sprayed concrete applicators beginning its journey in typical alpine conditions by a Garavanta cableway to its work-place